This invention relates to a method and apparatus for providing a continuous indication of the interior condition of an enclosed fluid container such as a water heater tank having a cathodic corrosion protection means therein.
Containers for fluid such as water heater tanks commonly have a steel housing to provide an enclosed chamber for holding the fluid. The interior surface of such steel housing is subject to corrosion due to contact with the fluid. In order to prevent the corrosion usually the interior surface is lined with a corrosion resistant material such as glass. However, even in such glass lined housing, there is still some bare iron surface which is exposed to the fluid in the container.
The bare iron interior surface in a glass lined or unlined water heater tank is subject to a corrosive action manifested by the existence of a galvanic electromotive force generated between different points on the iron surface and the free ions in the water. The iron surface is anodic with respect to the water; this causes the iron to dissolve gradually. Such corrosion of the iron interior surface of the water heater tank can be prevented by mounting in the tank a metal element which is strongly anodic with respect to all the bare iron surface. The anodic metal element is electrically connected to the iron interior surface so that it forms a cell with the iron surface and the water serves as the electrolyte of such cell. The iron interior surface becomes cathodic with respect to the metal element and the anodic metal element will dissolve in the process instead of the iron interior surface. Such anodic metal element is referred to as a sacrificial anode because it is sacrificed in the process such that there is no corrosion of the iron interior surface. Magnesium may be used as a sacrificial anode in water heater tanks because it does not release any toxic material into the water in the process.
Since the sacrificial anode is depleted in the cathodic protection process, it is essential to inspect it from time to time to see if it has been depleted to an unsatisfactory state or if it has depleted completely or there is such an excess amount of liming in the tank that the cathodic protection process is no longer in operation. Liming is the deposition or accumulation of a layer of hard calcareous material in the tank. However, as water heater tanks all have a fully enclosed structure, visual inspection of the sacrificial anode and the interior of the tank is not possible. Removal of the sacrificial anode for inspection may not be safely or conveniently made by the user since the anode must be mounted carefully and the temperature and pressure in the tank are hazardous if proper precautions are not adopted in removing the anode. Furthermore, the frequency of the removal of the anode for inspection may not be easily established as it varies according to variable conditions such as the mineral content in the water supply in a particular district and the rate of water flow according to the amount of water used by the user.